The present invention relates to improvements to airborne optical sighting devices which can move at high speed.
Fluids present a resistance to the movement of bodies immersed in them. This resistance is essentially a function of the type of fluid, the relative speed of the body in relation to the fluid and the shape of the body. It results, in particular, to a consumption of energy. Thus, for aircraft which must move at high speed, it is necessary to give the fuselage and the various projecting parts streamlined forms, called aerodynamic forms, so as to present the least possible resistance to forward movement. A body with poor streamlining, for example one which has slope discontinuities, causes an increase in the drag which essentially results from the occurrence of zones of broken-down flow, in which eddies and vortices arise.
Among the projecting elements are radomes or transparent fairings for housing various optical sighting systems. By optical sighting are meant both laser or other emitter systems and photoreceiver systems. These systems can serve, in particular, for target reconnaisance or for missile guidance; they are accommodated on the aircraft or arranged in a nacelle (called a "pod") which is usually fastened to the lower wing surface of the aircraft.
Because of the maneuverability of the aircraft, especially of the fighter type, optical sighting becomes necessary over a considerable field extending from the front of the aircraft to the rear and over the sides. Thus, the angular extent of the field in the longitudinal plane of the aircraft can be, for example, 120.degree. or more. A good compromise is obtained, at the present time, when optical transmitting and/or receiving means are arranged in the nose of a pod. With this configuration, as in the case of the radomes and fairings already mentioned, downward and rearward sighting makes it necessary to ensure that the beam assumes slightly inclined positions relative to the transparent wall through which it passes over a great length. In proportion to the increase in speed, however, this wall becomes the location for zones of turbulent flows which impede the passage of the light radiation, and this results in a poor quality of the video image detected.
Moreover, if the speed parameter is considered, there is no doubt that the thicknesses of these turbulent zones increase with the speed, and when the transsonic range is reached interactions with shock waves intensify these disturbances even further.
The object of the invention is to overcome such disadvantages by reducing these turbulence phenomena as much as possible at the level of the transparent wall, in order to maintain the convenience of optical sighting.
It is known, in aeronautics, to interpose a connecting piece between an air scoop and the wing or fuselage, the front shape of this connecting piece being designed to eliminate low-energy layers of air which are generated along the wall upstream of the air scoop and which would impair the performances of the engine. This piece has a front profile in the form of a stem-post; it forms a limiting-layer trap and thus ensures the supply of air to the engine with a sufficient energy. By limiting layer is meant the fluid thickness which rests against a wall and in which the viscous effects are revealed as a result of the movement of the body.
This solution is adopted here to perform a different function, that is to say to reduce the thickness of the limiting layer along the optical walls as much as possible, so that optical sighting, particularly towards the rear, can be carried out under good conditions.